This invention relates to single-mode power sources and more particularly to sources for delivering single-mode power to multi-channel devices.
Erbium-doped amplifying structures EDFAs are capable of providing large optical bandwidth channel capacity and low noise properties but require an optical source of power pump whose wavelength must lie within a spectral region that is efficiently absorbed by either Er3+ or co-doped Yb3+ ions in the amplifying structure, i.e., 900 to 980-nm or 1450 to 1480-nm, respectively. To achieve efficient coupling between the laser source and the EDFA through a single-mode fiber xe2x80x9cpath cordxe2x80x9d requires that the pump source be derived from a laser having a single spatial mode.
EDFA pump laser sources are typically derived from frequency stabilized single-mode fiber-coupled laser diodes c.f., U.S. Pat. No. 6,044,093 which use a fiber Bragg grating FBG that is highly transmissive at the C-band signal wavelength between 1530 and 1560-nm, and highly reflective at the pump wavelength, typically either 980 or 1480-nm. These devices have several drawbacks: They require frequency stabilization; they are relatively expensive and difficult to manufacture; and they have an inherent output power limitation due to the relatively low diode facet damage threshold.
In our copending application Ser. No. 09/734,112 filed Dec. 11, 2000 and entitled xe2x80x9cSingle Mode Pump Power Sourcexe2x80x9d, we describe an EDFA scalable high power pump source which converts the high-power, multimode output from a wide-stripe laser diode into a single-mode source of light within the pump band for either the Yb3+ or Er3+ ions of an EDFA.
Both the conventional fiber-coupled frequency-stabilized pump diodes and our above-mentioned micro-laser mode converter source are, however, single channel devices which provide pump power only to a single EDFA channel. Accordingly, a set of drive electronics as well as provision for cooling must be provided for each channel. In optical networking systems, where there are many tens or hundreds of EDFA""s, this entails considerable cost and space. Accordingly, there is a need for a compact and efficient integrated pump source that is capable of delivering the requisite optical pump power to multiple EDFA channels.
In accordance with the principles of our invention, a compact and efficient source of single-mode power for driving multiple EDFA channels employs a multimode source, such as a high-power laser diode bar, to pump an Nd3+ doped region defined in a cavity of a monolithic crystal structure. The axial length L1 of the doped region is chosen to optimize energy absorption from the multimode source while minimizing resonant re-absorption loss to unpumped Nd3+ ions. The next proximal cavity length L2 is an undoped region whose length is chosen to optimize the lowest order or fundamental spatial mode (xe2x80x9cmode 9xe2x80x9d) of the cavity. Advantageously, multi-parameter numerical optimization techniques may be employed in which the parameter set (e.g., doped length, L1, doping concentration, pump beam spot size, micro laser cavity length, and output coupler reflectivity) is varied to determine the overall optimal length L1opt.